The present invention relates to a phase-shifting mask and a method of manufacturing an electronic device using the mask. More particularly, it relates to formation of contact holes arranged at a high density in manufacturing a semiconductor integrated circuit device.
A dynamic random access memory (hereinafter referred to as the DRAM) functioning as a technology driver for a semiconductor integrated circuit device has played a significant role in promoting refinement and high integration of elements in a semiconductor integrated circuit device. Now, mass production of a DRAM of 64 Mbits designed in accordance with a 0.25 .mu.m rule has been realized. Furthermore, production of a DRAM of 1 Gbit designed in accordance with a 0.18 .mu.m rule has been expected.
In a DRAM of 1 Gbit, it is necessary to form each memory cell in an area of approximately 0.3 .mu.m.sup.2, and hence, there is a demand for a cell pitch of 0.4 .mu.m. A memory cell of an 8F.sup.2 structure used in a memory includes one storage node contact in each cell. Therefore, it is necessary to adopt a technique to form a contact hole with a diameter of approximately 0.2 .mu.m with a pitch of 0.4 .mu.m or less.
As a technique to improve resolution in patterning contact holes through exposure, an attenuated phase-shifting mask is proposed. In this technique, a semi-transparent film is formed in a field region of a photomask, thereby shifting the phase of the field region by 180.degree. from the phase of a transmitting hole region. Thus, interference is caused between transmitted light of these regions, so as to sharply define a pattern edge. By adopting this technique, an isolated contact hole with a diameter of approximately 0.2 .mu.m can be formed. In this technique, however, it is difficult to separate contact holes from one another when the pitch therebetween is smaller than approximately 0.5 .mu.m.
Therefore, use of an alternating phase-shifting mask having a higher degree of interference has been proposed for formation of contact holes with a smaller pitch (Japanese Laid-Open Patent Publication No. 62-50811). An alternating phase-shifting mask is a photomask provided with a phase shifter to every other transmitting areas for defining contact holes to be formed, so that a phase difference of 180 degrees can be caused between light transmitted by adjacent patterns. Such a phase shifter is generally formed by etching a photomask plate. By adopting this technique, a contact hole with a pitch of approximately 0.3 .mu.m can be formed.
The aforementioned phase-shifting mask is effective in forming a contact hole array where contact holes are periodically two-dimensionally arranged in an array of rows and columns. However, it is difficult to apply the phase shifting mask to contact holes to be formed in positions shifted from the regular matrix positions.
When a DRAM memory cell with an area of 0.3 .mu.m.sup.2 or smaller is to be formed, it is preferred that metal plugs are formed in a storage node contact hole and a bit line contact hole as is disclosed in, for example, IEDM '96 Technical Digest, p. 593. When these two types of contact holes are to be simultaneously formed, the storage node contact holes are regularly arranged in an array of rows and columns, but the bit line contact holes are disposed in positions shifted by a 1/2 pitch. Accordingly, it is difficult to form these contact holes at the same time by using the alternating phase-shifting mask. Therefore, after the storage node contact holes are formed by using the alternating phase-shifting mask, the bit line contact holes are required to be separately formed by using an attenuated phase-shifting mask or the like. As a result, discrepancy in locations of these contact holes, i.e., relative placement errors, cannot be avoided, which obstructs the development of refined memory cells.